Isoniazid appears to block the introduction of a double bond into the delta 5 position of saturated C22, C24, and C26 fatty acids. This probably leads to the inhibition in the synthesis of long-chain fatty acids (up to C56) and mycolic acids in Mycobacterium tuberculosis H37Ra. We have characterized many of these long-chain fatty acids and we shall continue this effort. We shall also isolate, purify and determine the structures of the mycolic acids of the H37Ra strain. We shall study the enzyme systems for the synthesis of these long-chain fatty acids. The first enzyme to be examined will be the desaturase system. BIBLIOGRAPHIC REFERENCES: Ribi, E., K. Takayama, K. Milner, G.R. Gray, M. Goren, R. Parker, C.L. McLaughlin, and M. Kelly, Regression of tumors by an endotoxin combined with trehalose mycolates of differing structure. Cancer Immunol. Immunother. 1, 265-270, 1976. Takayama, K., and E.L. Armstrong, Metabolic role of free mycolic acids in Mycobacterium tuberculosis H37Ra, J. Bacteriol. 130, 569-570, 1977.